1. Field of the Invention
The present invention relates to a gas separation/extraction apparatus for separating a desired gas from air or various other gas mixtures by pressure swing adsorption and a balance valve therefor.
2. Description of the Related Art
FIG. 11 shows a gas separation/extraction apparatus for separating oxygen from air by pressure swing adsorption. This apparatus has previously been proposed by the inventor hereof.
As shown in FIG. 11, the gas separation/extraction apparatus comprises gas separation tank 1, first three-way solenoid valve 2, free-piston pump unit 3, and second three-way solenoid valve 4.
Gas separation tank 1 has inlet port 6 for air as a gas mixture, outlet port 7 for separated oxygen, and exhaust port 8 for waste gas. The tank contains adsorbent 10 for adsorbing nitrogen under pressure.
In free-piston pump unit 3, as shown in FIGS. 12A and 12B, free piston 15, urged by means of spring 14, is slidably housed in cylinder 13 which communicates with suction port 11 and discharge port 12. Electromagnet 16 is provided on the outer periphery of cylinder 13. It attracts piston 15 against the urging force of spring 14. In pump unit 3, as shown in FIG. 12A, when electromagnet 16 is actuated by means of one half wave from a commercial AC power source so that piston 15 is attracted in the left direction in FIG. 12A, resisting the urging force of spring 14, suction valve 17 is opened to introduce air into cylinder chamber 18. Then, in response to the other half wave for the change of the current direction, the drive of electromagnet 16 is stopped by the agency of diode 16A. At this time, free piston 15 is slid in the right direction in FIG. 12B by the urging force of spring 14, thereby compressing the air in chamber 18, and opening discharge valve 20 to cause the air to be discharged through discharge port 12. In this gas separation/extraction apparatus, free-piston pump unit 3 doubles as a vacuum pump and a compressor.
In the apparatus described above, first three-way solenoid valve 2 has a passage, which connects exhaust port 8 of gas separation tank 1 and suction port 11 of free-piston pump unit 3, and a passage through which air is introduced and led to port 11. Normally or in the OFF state as shown in FIG. 11, valve 2 is kept open so that air is introduced and led to port 11. In the ON state, the valve is switched to allow exhaust port 8 of tank 1 to communicate with suction port 11.
Second three-way solenoid valve 4 has a passage, which connects discharge port 12 of free-piston pump unit 3 and inlet port 6 of gas separation tank 1, and a passage through which port 12 opens to the outside air. Normally or in the OFF state, as shown in FIG. 11, discharge port 12 is caused to communicate with inlet port 6 of tank 1. In the ON state, the valve is switched to allow port 12 to open to the outside air.
Thus, by ON-OFF controlling first and second three-way solenoid valves 2 and 4 with a predetermined timing, decompression and pressurization of gas separation tank 1 can be repeatedly performed by means of free-piston pump unit 3. More specifically, tank 1 is pressurized and decompressed when solenoid valves 2 and 4 are OFF and ON, respectively.
The following is a description of the operation for separating and extracting oxygen in air by means of the gas separation/extraction apparatus of this type.
If free-piston pump unit 3 is first started with three-way solenoid valves 2 and 4 off, air introduced through first solenoid valve 2 is compressed in cylinder chamber 18 of pump unit 3 (FIG. 12B), and the compressed air flows through second solenoid valve 4 into gas separation tank 1, thereby pressurizing the tank. When the inside pressure of tank 1 attains a predetermined level, nitrogen in the air is adsorbed by adsorbent 10, so that tank 1 is filled with enriched oxygen. By opening outlet port 7 to take out the oxygen gas in tank 1 in this state, oxygen in the air can be separated and extracted.
Subsequently, outlet port 7 of gas separation tank 1 is closed, first and second three-way solenoid valves 2 and 4 are switched on, and free-piston pump unit 3 is started. Thereupon, the residual gas in gas separation tank 1 is sucked into cylinder chamber 18 (see FIG. 12A). By the action of pump unit 3, the sucked residual gas is guided through discharge port 12 to solenoid valve 4, and then discharged into the outside air. Thus, tank 1 is gradually decompressed. When the pressure of the tank is reduced to a predetermined level, the nitrogen having so far been adsorbed by adsorbent 10 starts to be released. The residual gas, consisting mainly of the released nitrogen gas, is discharged through second solenoid valve 4 into the outside air. When valves 2 and 4 are switched off after the discharge of the residual gas is finished, free-piston pump unit 3 operates as a compressor, so that air is resupplied into gas separation tank 1 under pressure.
Thus, oxygen in air is separated and extracted in succession by switching three-way solenoid valves 2 and 4 and repeating the pressurization and decompression of gas separation tank 1.
This trial machine, however, has the following problems. Since free piston 15 of free-piston pump unit 3 is designed so as to run against head-side end face 21 of cylinder 13, abutting portions of the two members may be damaged, and a great impactive noise is produced by the collision. These situations are attributable to the following causes. If solenoid valves 2 and 4 are simultaneously turned off to switch the operation mode of pump unit 3 from vacuum pump operation to compressor operation, cylinder chamber 18 is connected to gas separation tank 1 under vacuum, and therefore, is decompressed. As a result, a bumper effect (air cushion effect) against the compressive movement of free piston 15 is reduced.
In order to solve these problems, in the improved trial machine, a branch portion is provided in a passage extending from three-way solenoid valve 4 to gas separation tank 1, as shown in FIG. 11, and two-way solenoid valve 5, which has an orifice and a selector valve, is disposed at the end portion of branch passage 22. Normally or in the OFF state, valve 5 closes the end portion of passage 22. In the ON state, valve 5 switches passage 22 to the orifice side, thereby making the passage open to the outside air so that the pressure inside cylinder 13 of free-piston pump unit 3 is restored to the level of the atmospheric pressure.
Constructed in this manner, however, the improved trial machine requires a complicated control circuit, as shown in FIG. 13. In the circuit diagram of FIG. 13, symbols S1, S2 and S3 designate solenoid valves, T1, T2 and T3 designate timers used to determine the ON-operation period of valves S1, S2 and S3, respectively, and R1, R2 and R3 designate relays for turning on and off valves S1, S2 and S3, respectively. Subscripts 1, 2 and 3 attached to characters S, R and T are indicative of circuits for operating first and second three-way solenoid valves 2 and 3 and two-way solenoid valve 5, respectively. Solenoid valves S1, S2 and S3 are timing-controlled in accordance with the time chart of FIG. 14. Thus, when the operation mode is switched from decompression to pressurization (or when solenoid valves 2 and 4 are turned off), the timing for the switching is shifted so that S2 is turned off with a delay of time t after S1. Two-way solenoid valve 5 or S3 is turned on within time t. Thus, even though first three-way solenoid valve 2 is turned off to introduce air, the compressed air is discharged through second three-way solenoid valve 4 into the atmosphere for the period of time t, without being immediately fed into gas separation tank 1. Meanwhile, two-way solenoid valve 5 is opened to allow air to be introduced through an orifice passage therein within the period of time t. The introduced air is temporarily fed into tank 1 under vacuum through branch passage 22, thereby increasing the pressure of the tank, and solenoid valve 5 is opened thereafter. Then, second three-way solenoid valve 4 is turned off to connect discharge port 12 of free-piston pump unit 3 and inlet port 6 of tank 1.
If cylinder chamber 18 is caused to communicate with gas separation tank 1 whose internal pressure thus is increased, the pressure difference between the head and tail sides of free piston 15 is small, and the bumper effect of chamber 18 against the compressive motion of the piston is slightly enhanced. Accordingly, free piston 15 is somewhat restrained from running against head-side end face 21. Thus, the problems of the damage to the abutting portions of end face 21 and piston 15 and the impactive noise can be solved in a way.
In order to provide the aforementioned improved trial machine with two-way solenoid valve 5 having the orifice passage therein, however, the machine must be furnished with an additional electric circuit for driving valve 5. Thus, the circuit configuration is further complicated, and equipment costs are high.
In the arrangement described above, moreover, air is guided into gas separation tank 1 through the orifice. In raising the vacuum pressure inside gas separation tank to the level of the atmospheric pressure, however, the conventional arrangement requires too much time to increase the pressure to a level such that free piston 15 cannot run against head-side end face 21. Thus, the operating efficiency is not very high.
If ON-operation time t for two-way solenoid valve 5 is made unduly high, furthermore, the operating efficiency is lowered corresponding. If time t is too short, on the other hand, free piston 15 inevitably runs against end face 21. Thus, it is difficult to set time t properly, and therefore, designing of the circuits for driving the apparatus requires scrupulous care and a lot of time.